1. Field of the Invention
This invention relates generally to a dual frequency antenna system and, more particularly, to a satellite antenna system employing a dual frequency polarizer and a dual band orthomode transducer that separates a dual frequency signal having different polarizations.
2. Discussion of the Related Art
Various communications systems, such as certain telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, that retransmits the signal to another satellite or to the Earth as a satellite downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bands. For example, the uplink signal may be transmitted at 30 GHz band and the downlink signal may be transmitted at 20 GHz band. The satellite is equipped with antenna systems including a number of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth.
For some of these satellite communications systems, one antenna system is provided for receiving the uplink signals and another antenna system is provided for transmitting the downlink signals. Each antenna system typically employs an array of antenna feed horns and one or more reflectors to collect and direct the signals. The uplink and downlink signals are circularly polarized so that the orientation of the reception antenna can be arbitrary relative to the incoming signal. To provide signal discrimination, one of the signals may be left hand circularly polarized (LHCP) and the other signal may be right hand circularly polarized (RHCP), where the signals rotate in opposite directions. Polarizers are employed in the antenna systems to convert the circularly polarized signals to linearly polarized signals suitable for propagation through a waveguide with low signal losses.
Because there are important weight and real estate limitations on a satellite, it is desirable to use the same antenna system for both transmitting the downlink signal and receiving the uplink signal. Because the uplink and downlink signals are at different frequency bands, the feed horns would have to be designed to transmit and receive the signals at both the uplink and downlink frequency bands. It would also be necessary to employ a dual band polarizer that could effectively convert the downlink signal from a linearly polarized signal to a circularly polarized signal and convert the uplink signal from a circularly polarized signal to a linearly polarized signal. However, known polarizers can only be optimized for a single frequency band, making them unsuitable for polarizing signals of different frequencies.
Known dual frequency antenna networks of the type being described herein sometimes employ a turnstile junction to equally divide the signal into orthogonal components. A discussion of turnstile junctions can be found in U.S. patent application Ser. No. 09/494,612, titled xe2x80x9cWideband TE11 mode Coaxial Turnstile Junction,xe2x80x9d and assigned to the assignee of this application.
What is needed is an antenna system and associated feed network capable of transmitting a satellite downlink signal and receiving a satellite uplink signal, that is able to effectively provide polarization conversion in two separate frequency bands. It is therefore an object of the present invention to provide such an antenna system.
In accordance with the teachings of the present invention, an antenna system is disclosed that employs antenna elements that provide both transmit and receive functions. Signals received by each antenna element are directed to a dual band polarizer that converts the signals to linearly polarized signals. Signals to be transmitted by each antenna element are converted to circularly polarized signals by the polarizer. Depending on the orientation of the dual band polarizer and whether the received signal is LHCP and/or RHCP, the polarizer will convert the circularly polarized signal to a vertically and/or horizontally linearly polarized signals. Likewise, linearly polarized signals received by the polarizer will be converted to LHCP and/or RHCP signal depending on the orientation of the polarizer with respect to the OMTand whether the linearly polarized signal is vertically or horizontally linearly polarized.
A dual-band orthomode transducer is employed to direct the transmit signals to the polarizer and receive the received signal from the polarizer. The transducer receives separate linearly horizontally polarized signals and/or linearly vertically polarized signals, and couples them together for the transmit signal. The transducer receives the receive signal and separates it into its linearly horizontally polarized components and/or linearly vertically polarized component at one and/or two ports of the transducer. In one embodiment, a high pass filter is used to help separate the receive signals, and a low pass filter is used to help separate the transmit signals.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.